Although lightning strikes high voltage power lines very frequently, lightning generally causes a high voltage surge within a short distance, say around 200 meters, of the impacted site. Consequently, relatively few end users of electronic equipment are affected. Transient Voltage Surge Suppressors (TVSS) devices protect against such rare but damaging voltage surges. On the other hand, every lightning strike on a power line or other power system fault causes a short-duration voltage sag that lasts typically less than six cycles, impacting customers up to 200 miles away. As a result, end users of electronic equipment such as computers, televisions, medical equipment, etc., are likely to experience voltage sags much more frequently than voltage surges.
In addition, during start up of electronic equipment, there is often an inrush current that may cause damage to electrical components. To limit the damaging effects of such inrush currents, a thermistor may typically be employed that limits inrush current upon startup of electronic equipment. Specifically, a thermistor might be operated to inject an impedance such as a resistance into a power circuit to limit the inrush current when the thermistor is cool at startup of the electronic equipment. However, after startup, a thermistor is heated, thereby reducing the inserted resistance. As a result, the thermistor no longer functions as a current inrush limiter. This can be problematic due to the common occurrence of voltage sags. At the end of a voltage sag that occurs after start up, the AC line voltage may abruptly return to normal potentially causing a large current surge that is not limited due to the fact that the thermistor is disengaged after start up.